A design processing system such as a computer-aided design (CAD) system or the like is used in the design of circuit boards such as printed circuit boards and the like. It has become usual for arrangements of wiring and the like to be determined with consideration for the positions and sizes of components to be mounted on the circuit boards. As the signal clocks of electronic equipment become faster, it is also common to design circuit boards with consideration for the generation of electromagnetic field noise.
For example, the signal waveform of a high-speed signal clock may be distorted by electromagnetic field noise emitted by electronic equipment, and this distortion may affect operations. Electromagnetic field noise is caused by currents flowing in loops formed by combinations of the paths of currents flowing as signals and the paths of return currents flowing to ground. A field intensity causing electromagnetic field noise tends to be larger when the area of a loop is larger. Therefore, electromagnetic field noise may be suppressed by reducing the areas of the loops. Thus, electromagnetic field noise may be suppressed by keeping the paths of currents that flow as signals and the paths of return currents close together.
A number of technologies that suppress electromagnetic field noise in circuit boards are known. For example, a technology is known in which overlap regions are found, in which regions whose widths are the width of a target signal line magnified to a pre-specified width overlap with ground patterns, detour paths are found for discontinuity portions of the overlap regions, and the suitability of the paths of return currents is judged. Another technology is known that evaluates the acceptability of a printed circuit board design, with the path of a return current being defined by the shortest paths joining vias on a signal line with vias that connect ground patterns within a certain distance from the signal line. Still another technology is known that extracts vias connecting the ground patterns of a target signal with ground patterns in adjacent layers and that, where the path of the return current is broken, finds a detour through the vias.